Marine microalgae, also called phytoplankton, are crucial contributors to global carbon cycle and oceanic ecology. Indeed, they perform photosynthesis, the light-driven capture on CO2 into biomass, which is a carbon sink and a founding reaction of marine food webs. More specifically, coccolithophores are an important group of unicellular algae because they are very abundant and produce an “exoskeleton” made of calcium carbonate.
In contrast with most eukaryotes (e.g. animals and land plants), coccolithophores thrive through a haplo-diplontic life cycle: both haploids and diploids (equivalent to gametes and egg cells, respectively) can undergo asexual divisions (mitosis). Moreover, within a given species, haploid and diploid cells generally form distinct extra-cellular calcareous plates. In surveys of phytoplankton spatiotemporal distribution, abundance of coccolithophore’s haploid and diploid cells are generally contrasted. It has thus been proposed that haploids and diploids display different physiological features to colonize distinct ecological niches of the ocean. The aim of this project is to challenge this hypothesis.
Phytoplankton species have been relatively rarely studied in laboratory cultures because their growth is generally slow and requires highly specific conditions. I focus on the calcifying coccolithophore Calcidiscus leptoporus as a new alternative model specis and combine growth tests and measures of photosynthetic performance in strains from various locations around the globe. To gain insight at the molecular level, I also investigate the genome. Studying the life cycle of coccolithophore addresses both a fundamental topic and global ecological questions; indeed, human activities have multiple drastic effects on the ocean, and how this impacts phytoplankton, the first link in ecosystems food networks, is a major issue for biodiversity, society and economy.
